多线程 读写同步

多线程同步时   等待信号后  需要while条件判断 不能用if。 具体原因不确定 。

此文章说是因为可能有“惊群效应”。 

/* 等待缓冲区非空*/
    while (b->writepos == (b->readpos) )
    {
        pthread_cond_wait(&b->notempty, &b->lock);
    }

#include <stdio.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/syscall.h>
#define BUFFER_SIZE 16 // 缓冲区数量

// gcc example.c -lpthread -o example

struct prodcons
{
    // 缓冲区相关数据结构
    int buffer[BUFFER_SIZE]; /* 实际数据存放的数组*/
    pthread_mutex_t lock; /* 互斥体lock 用于对缓冲区的互斥操作 */
    int readpos, writepos; /* 读写指针*/
    pthread_cond_t notempty; /* 缓冲区非空的条件变量 */
    pthread_cond_t notfull; /* 缓冲区未满的条件变量 */
};
/* 初始化缓冲区结构 */
void init(struct prodcons *b)
{
    pthread_mutex_init(&b->lock, NULL);
    pthread_cond_init(&b->notempty, NULL);
    pthread_cond_init(&b->notfull, NULL);
    b->readpos = 0;
    b->writepos = 0;
}
/* 初始化缓冲区结构 */
void release(struct prodcons *b)
{
    pthread_mutex_destroy(&b->lock);
    pthread_cond_destroy (&b->notempty);
    pthread_cond_destroy (&b->notfull);
    b->readpos = 0;
    b->writepos = 0;
}
/* 将产品放入缓冲区,这里是存入一个整数*/
void put(struct prodcons *b, int data)
{
    pthread_mutex_lock(&b->lock);
    /* 等待缓冲区未满*/
    if ((b->writepos + 1) % BUFFER_SIZE == b->readpos)
    {
        pthread_cond_wait(&b->notfull, &b->lock);
    }
    /* 写数据,并移动指针 */
    b->buffer[b->writepos] = data;
    b->writepos++;
    // if (b->writepos >= BUFFER_SIZE)
    //     b->writepos = 0;
    //if ((b->writepos + 1) % BUFFER_SIZE == b->readpos){
        b->writepos = (b->writepos ) % BUFFER_SIZE;
    //}
    /* 设置缓冲区非空的条件变量*/
    pthread_cond_signal(&b->notempty);
    pthread_mutex_unlock(&b->lock);
} 
/* 从缓冲区中取出整数*/
int get(struct prodcons *b)
{
    int data;
    pthread_mutex_lock(&b->lock);
    /* 等待缓冲区非空*/
    while (b->writepos == (b->readpos) )
    {
        pthread_cond_wait(&b->notempty, &b->lock);
    }
    // printf("***%d %d \n", b->writepos,b->readpos);
    /* 读数据,移动读指针*/
    data = b->buffer[b->readpos];
    b->readpos++;
    // if (b->readpos >= BUFFER_SIZE)
    //     b->readpos = 0;
    //if ((b->readpos + 1) % BUFFER_SIZE == b->writepos){
        b->readpos = (b->readpos) % BUFFER_SIZE;
    //}
    /* 设置缓冲区未满的条件变量*/
    pthread_cond_signal(&b->notfull);
    pthread_mutex_unlock(&b->lock);
    return data;
}
 
/* 测试:生产者线程将1 到10000 的整数送入缓冲区,消费者线
   程从缓冲区中获取整数,两者都打印信息*/
#define OVER ( - 1)
struct prodcons buffer;
void *producer(void *data)
{
    int n;
    for (n = 0; n < 50; n++)
    {
        printf("%d --->\n", n);
        put(&buffer, n);
    } put(&buffer, OVER);
    put(&buffer, OVER);
    put(&buffer, OVER);
    put(&buffer, OVER);
    return NULL;
}
 
void *consumer(void *data)
{
    int d;
    while (1)
    {
        d = get(&buffer);
        if (d == OVER)
            break;
        printf("--->%d %d \n", d,pthread_self());
    }
    return NULL;
}
 
int main(void)
{
    pthread_t th_a, th_b, th_c, th_d;
    void *retval;
    init(&buffer);
    /* 创建生产者和消费者线程*/
    pthread_create(&th_a, NULL, producer, 0);
    pthread_create(&th_b, NULL, consumer, 0);
    pthread_create(&th_c, NULL, consumer, 0);
    pthread_create(&th_d, NULL, consumer, 0);
    /* 等待两个线程结束*/
    pthread_join(th_a, &retval);
    pthread_join(th_b, &retval);
    pthread_join(th_c, &retval);
    pthread_join(th_d, &retval);
    release(&buffer);
    return 0;
}

　　

https://www.cnblogs.com/x_wukong/p/7909895.html